A polymer having such a function as optical compensation can be obtained by polymerizing a polymerizable compound having a liquid crystal phase. The function is exhibited by fixing the alignment of the liquid crystal molecules through polymerization. Several polymerizable compounds are developed for utilizing such the function of polymerizable compound. However, the function may not be sufficiently exhibited with only one kind of polymerizable compound. Accordingly, such attempts have been made that a composition is prepared with plural kinds of polymerizable compounds, and the composition is polymerized to provide a polymer (see JP-H10-319408-A (GB 2324382A), JP-2004-198478-A, JP-2002-243942-A and JP-2005-196221-A).
In this specification, the fact that a liquid crystal skeleton has an alignment state, such as a homogeneous alignment, a tilted alignment, a homeotropic alignment, a twisted alignment or the like may be referred to as “having a homogeneous alignment”, “having a tilted alignment”, “having a homeotropic alignment”, “having a twisted alignment” or the like, respectively. For example, a liquid crystal film having a homeotropic molecular alignment, i.e., a homeotropically-aligned liquid crystal film, may be referred to as a liquid crystal film having a homeotropic alignment or a homeotropically-aligned liquid crystal film.
A polymer having a homeotropic alignment has an optical axis in the nz direction, and the refractive index in the direction of the optical axis is larger than the refractive index in the direction perpendicular to the optical axis. Accordingly, the polymer is classified into a positive C plate in terms of optical ellipsoid. The positive C plate can be applied, by combining with a film having an other optical function, to optical compensation of a liquid crystal mode with a horizontal alignment, i.e., a so-called IPS (in-plane switching) mode, and the like, such as improvement in viewing angle characteristics of a polarizing plate (as described in M. S. Park, et al., IDW, '04, FMCS-4, M. Nakata, et al., SID, '06, P-58, K. J. Kim, et al., SID, '06, Digest, p. 1158-1161, WO 05/038517 A1 and US 2006/0182900 A1).
In the aforementioned use, there are case where the polymerizable liquid crystal material is laminated on a glass substrate, a glass substrate having coated on the surface thereof a plastic thin film (such as an overcoated film formed on a color filter), a color filter substrate (see JP-2005-164962-A) or a plastic substrate. Examples of a material used as the plastic substrate include such polymers as TAC (triacetyl cellulose), polycarbonate, PET (polyethylene terephthalate) and a cycloolefin resin.
The inventors have found a polymerizable liquid crystal composition that can form a uniform homeotropic alignment on a glass substrate or a plastic substrate with good adhesion without an alignment film formed on the substrate (see JP-2006-126757-A and JP-2007-16213-A). However, the composition may sometimes be difficult to control the optical characteristics thereof, particularly birefringence (Δn) and thus leave room for improvement. As a method for controlling birefringence, utilization of a compound having an aromatic ring in the short axis direction of mesogen (a compound having a triptycene ring) has been proposed, but it is still difficult to form a uniform homeotropic alignment only by the proposal (see JP-2006-111571-A).